150 ADVANCED PHYSIOLOGY 



The Portal Blood System. The portal blood system has 

 already been fully described (see page 118). Briefly sum- 

 marized, the portal vein begins in the capillaries of the in- 

 testine, stomach and spleen; the blood from all these organs 

 runs together to form the portal vein proper, which enters 

 the liver and there breaks up into capillaries, for purposes 

 already mentioned (shown also in Fig. 81). The liver, then, 

 receives venous blood from the portal vein, and arterial 

 blood from a branch of the aorta as it runs down through the 

 abdomen. 



GENERAL SUMMARY OF THE CIRCULATION 



The circulation of the blood is a double one, one half going 

 from the right ventricle through the lungs and back to the left 

 ventricle; the other half from the left ventricle through the 

 body and back to the right ventricle. The circulation through 

 the lungs is called the pulmonary circulation; that through the 

 rest of the body is the systemic circulation. During its entire 

 passage around the body the blood as blood never leaves th( 

 arteries, capillaries and veins (except in the spleen) . Remember- 

 ing that the right and left sides of the heart are entirely sepa- 

 rated, it is plain that the blood after leaving the left ventricle 

 of the heart, traverses the body, returns to the right side ot 

 the heart, goes thence to the lungs and so finally returns to 

 the left side of the heart whence it started. It has however 

 traversed no area twice even though it has been twice to the 

 heart. 



STRUCTURE OF BLOOD VESSELS 



If an artery be taken from an animal's body it will be found 

 that it is not a limp tube, but rigid enough to keep its shape 

 even when empty. The tissues which make up an artery are 

 arranged in three chief layers; Fig. 82. Next to the cavity 



